This invention relates to an apparatus for conducting Raman spectroscopy.
A Raman spectrum generally corresponds to frequencies of molecular vibrations and therefore can be related directly to molecular structure. In Raman spectroscopy, monochromatic light (excitation light) is generally directed onto a sample. Typically, this monochromatic light is a single laser line. Most of the light scattered off the sample will be at the same wavelength as this laser line (Rayleigh scattering), but a portion of the light scattered off the sample will be scattered at wavelengths containing the sum or difference of the excitation and molecular vibrational frequencies (Raman scattering).
Optical fibers have been advantageous in Raman spectroscopy. When optical fibers are used, light from a laser can be delivered to a sample over one fiber. After passage through the sample, the light scattered from the sample is collected by one or more other fibers and directed into a wavelength selective light detector, i.e., a spectrometer. The advantages of using optical fibers in Raman spectroscopy include sampling remotely from the spectrometer, sampling in a hostile environment and connecting several sampling systems to a single detector. The primary disadvantage of using optical fibers is that Raman spectra may be generated from the optical fiber material itself, interfering with the Raman spectra of the sample. For example, if a silica-core fiber is used, the transmitted light will generate Raman spectra from the silica in the fiber. Part of this silica based Raman spectra, along with part of the transmitted light, may be scattered by the sample and enter the collecting optical fiber. The collected excitation light will generate additional silica Raman light as it traverses the collecting optical fiber. This silica based Raman spectra will be directed back to the spectrometer along with the sample Raman spectra, thereby interfering with the analysis.
The fiber optic probe for Raman analysis of U.S. Pat. No. 4,573,761 issued to McLachlan, Jewett and Evans on Mar. 4, 1986 was a substantial advance in the art of Raman spectroscopy using fiber optic probes. The probe of the '761 Patent allowed excitation light scattered from the sample to be directed back to the detector by way of a silica based optical fiber. However, this light generated interfering silica Raman spectra convolved with the sample Raman spectra. This interference is most serious when the sample is a turbid liquid, a solid, or solid particles, because such samples tend to scatter substantial amounts of light.
The fiber optic probe for Raman analysis of U.S. Pat. No. 5,112,127 issued to Carrabba and Rauh on May 12, 1992 was a further advance in the art of Raman spectroscopy using fiber optic probes because a filter (element 44 of FIG. 1 of the '127 Patent) was positioned in the path of the Raman spectra before it enters the optical fiber connected to the spectrometer. The filter was an edge filter or a notch filter which blocked the laser wavelength but passed the Raman spectra and therefore eliminated the possibility of interfering silica Raman spectra on top of the sample Raman spectra. However, the probe of the '127 Patent exposed the filter to hostile sample conditions such as heat, which can deteriorate such filters.
It would be a further advance in the art of Raman spectroscopy fiber optic probes if the filter could be protected from such hostile sample conditions without introducing interfering Raman spectra.